Conventional types of image forming apparatus include an image forming apparatus which writes pictorial information into a photo-sensitive body using a laser beam. In recent years, in the image forming apparatus as described above, a plastic lens has been used for such purposes as cost reduction or weight reduction.
Also in association with popularization or expansion of application scope of image forming apparatuses such as a laser printer, a laser facsimile equipment and a laser copier, a demand for capability for constant magnification (accuracy in variable power) has been becoming increasingly stronger. However, in an image forming apparatus using a conventional type of plastic lens therein, although reduction of cost and weight is possible, state of the plastic lens changes due to such factors as change of environmental temperature or temperature inside the machine, sometimes a scanning position on an image surface on a photo-sensitive body changes, which causes a magnification error in the main scanning direction and makes it impossible to obtain a high quality image. Especially in a copier which is required to execute precise magnification or reduction of an image, the capability for constant magnification (accuracy in variable power) becomes lower, which is disadvantageous.